Kaolin products are commonly used by the paper industry to fill and coat paper and paperboard products. These kaolin coatings and fillers may serve to improve the quality of the paper product, for instance, in terms of texture and surface characteristics and resulting in improved printing quality. Kaolin products are also used, for example, in the paint, plastics, and ink industries.
Kaolin is available in various types, for instance, calcined and hydrous types. The type of kaolin is typically selected by the user based upon the desired end product. Untreated kaolin is commonly referred to as “hydrous” because it has not undergone heating to remove hydroxyl groups generally present in its natural structure. Calcined kaolin compositions may be prepared by heating a kaolin clay to a temperature of at least 400° C., for example, a temperature ranging from 400° C. to 1100° C. Depending on the calcination temperature and residence time, the kaolin product may be rendered substantially anhydrous (fully calcined) or partially anhydrous (partially calcined), in which case the kaolin may be referred to as a metakaolin. As used herein, the term “kaolin” is meant to refer to all types of kaolin, including partially calcined kaolin, fully calcined kaolin, and hydrous kaolin.
Kaolin products are often sold to a manufacturer in the form of a pulverized low bulk density powder. That powder may be difficult to handle using conventional shipping methods as they may be dusty, resulting in wasted product and environmental contamination. The powders may also require the use of sparger cars, which are bulk-hopper railroad cars equipped with valves that allow water to be introduced to the kaolin powder upon arrival at the customer's facility. The water may be injected into the railcar and the mixture of kaolin powder and water is agitated by means of turbulence. However, this shipment method is both expensive and inconvenient in terms of handling.
Kaolin products have also been shipped in the form of pre-mixed slurries to avoid the problems associated with shipping low bulk density powders. However, those slurry shipments require specialized storage, shipping, and handling equipment. Furthermore, kaolin slurries may comprise from about 25 wt % up to about 50 wt % water and, thus, the expense of shipping these large volumes of water may become cost prohibitive.
Efforts have been made in the prior art to increase the bulk density of kaolin powders by forming cakes or bricks of kaolin products. Those cakes may be formed by filtering slurries of acid-flocculated kaolin through filter cloths to form a cake and then drying the cake. The cake may then be broken into smaller chunks or lumps and shipped in this form. Once at the customer's facility, the lump kaolin may be mixed with water to form a slurry. However, this lump form of kaolin often requires the use of high shear energy to liberate particles, the use of chemicals to adjust pH to near neutral, and the use of dispersants to enable stable high solids mineral-water slurries. Thus, this method often proves expensive in terms of energy consumption and the additional material cost of chemicals and dispersants.
Alternative methods in the prior art include spray drying of kaolin powders, for instance, as described in U.S. Pat. No. 5,129,953, which appears to convert clay slurries into small, low-moisture kaolin spheres. While this process may be an improvement over lump kaolin processing in terms of make-down, spray drying often produces kaolin products that are dusty, e.g., products having a large number of particles falling in a size range of less than 200 microns, for example less than 50 microns and by further example less than 10 microns. Thus, spray dried products may be difficult to ship, much like the pulverized kaolin powder products.
Kaolin products may also be shipped in pelletized/agglomerated form, for example, as described in U.S. Pat. Nos. 4,246,220 and 5,364,579; however, pelletization typically involves addition of up to 40 wt % water to form acceptable pellets. Thus, pelletized kaolin products typically result in increased shipping costs or require evaporation of a portion of the water before shipment, which affects production costs. Pellets have also been made using binders other than water; however, those pellets are often more expensive to produce and also may present difficulties when the customer attempts to dissolve the pellets in water for the desired end use.
Finally, efforts have been made in the prior art, for instance, U.S. Pat. No. 5,328,506 and PCT Publication No. WO 97/42268, to improve the bulk density and flowability of kaolin agglomerates, e.g., to improve their resistance to compaction during shipment and/or storage. However, those agglomerates still do not exhibit sufficiently improved handling properties in systems such as large silos due to high fines content.
Thus, it would be useful to provide granulated kaolin compositions exhibiting at least one property chosen from improved material handling, low dusting, and easy make-down into a mineral-water slurry, thereby allowing for at least one of more cost effective production, shipment, and handling of the kaolin products.